1. Field of the Invention
This invention relates to a method for making a semiconductor apparatus including the step of crystallizing an insulating film made of a metal oxide and also to a method for making a thin film semiconductor substrate and a method for making a display apparatus, both using the semiconductor apparatus. The invention also relates to a semiconductor apparatus, a thin film transistor substrate and a display apparatus obtained by the methods mentioned above, respectively.
2. Description of the Related Art
In recent years, next generation displays have been extensively developed in the field of display apparatus, in which there is a demand for space-saving, high luminance, low consumption power and the like. For such display apparatus, attention has been drawn to organic EL (electroluminescence) displays using organic electroluminescent devices. This organic EL display has features in that a view angle thereof is wide because of the use of an organic electroluminescent device that is of a self-emitting type and power saving can be expected due to the unnecessity of backlight and that its response is high and the display can be made thin. Moreover, because a plastic substrate is used as a substrate for the purpose of making use of flexibility inherent to an organic electroluminescent device, attention has also been paid thereto as a apparatus having flexibility.
Among drive systems for the organic EL display, an active matrix system using a drive device of thin film transistors (TFT) is superior to conventional passive matrix systems with respect to response time and resolution. Thus, it has been accepted that the active matrix system is particularly suited for organic EL displays having such features as set out above.
The organic EL display of the active matrix type has an organic electroluminescent device having at least organic luminescent material and a drive substrate provided with a drive device (thin film transistor) driving the luminescent device. The organic EL display is so arranged that the drive substrate is bonded with a seal substrate so as to sandwich the organic electroluminescent device therebetween through a bonding layer.
The organic EL display of the active matrix type essentially includes, at least, a switching transistor controlling pixel contrast and a drive transistor controlling luminescence of the organic electroluminescent device. A gate electrode of the drive transistor is connected with a retention capacitor for retaining a charge in response to a response signal.
It is known that with a thin film transistor, when a voltage is continuedly applied to a gate electrode thereof, a threshold voltage is shifted. Moreover, the thin film transistor of the organic EL display is liable to cause the threshold shift because it is necessary to keep the state of current passage so far as the organic EL device is subjected to luminescence emission. If the threshold voltage of the drive transistor is shifted, an amount of current passing through the drive transistor varies, with the result that the luminescent elements constituting individual pixels change in luminance.
In recent years, in order to mitigate the threshold shift of this thin film transistor, there has been developed an organic EL display using a thin film transistor wherein the channel region is formed of a semiconductor layer of polysilicon (p-Si). However, in the polysilicon process, crystal grains existing in the channel are non-uniform in size, with the likelihood that transistor characteristics vary. Eventually, there arises a problem in that the emission luminance of the organic EL display differs on a pixel to pixel basis.
In order to suppress the variation of the transistor characteristics, there has been used a technique wherein silicon is finely crystallized to such an extent that crystal grains do not become non-uniform in size. Moreover there has also been used a technique wherein an amorphous silicon layer is formed on a polysilicon layer so as to suppress the variation (see, for example, Japanese Patent No. 2814319).
In this connection, however, the finely crystallized silicon layer is smaller in carrier mobility that a polysilicon layer. In order to obtain the same amount of luminescence or on current, a drive transistor used to control the luminescence of an organic electroluminescent device requires a high voltage, thus resulting in an increase in consumption power.
Magnitude Id of an on current passing through the formation of a channel in the drive transistor is expressed according to the following equation (1)Id=(1/2)μ(W/L)Cox(Vg−Vth)2  (1)In the equation (1), μ represents a carrier mobility in a reversed layer converted to a channel, W represents a gate width of the transistor, L is a gate length of the transistor, Cox represents a capacitance of a gate insulating film, Vg is a gate voltage, and Vth represents a threshold voltage.
According to the above equation (1), it will be seen that in order to obtain the same on current, it is important to increase the carrier mobility μ, gate width W, capacitance Cox of the gate insulating film or voltage difference (Vg−Vth), or to reduce the gate length L.
In order to increase capacitance Cox of the gate insulating film, it is necessary to increase a specific permittivity of the gate insulating film or decrease a physical film thickness of the insulating film. However, in view of the fact that the gate insulating film has a problem on leak current, a difficulty is involved in decreasing the film thickness. For increasing the specific permittivity of the gate insulating film, it is necessary to use a material having high permittivity. As a material whose specific permittivity is high, mention is made, for example, of metal oxides such as tantalum pentaoxide. Metal oxides such as tantalum pentaoxide, titanium oxide and the like exhibit a specific permittivity as high as about several times to about 100 times a silicon oxide (SiOx) film or a silicon nitride (SiNx) film as a hitherto employed gate insulating film. Thus, the use of these metal oxides as a gate insulating film enables the capacitance Cox of gate insulating film to be increased.
In general, metal oxides such as tantalum pentaoxide, titanium oxide and the like are formed as a film in an amorphous state according to a plasma CVD (chemical vapor deposition) method or a sputtering method. However, if tantalum pentaoxide or titanium oxide is in an amorphous state, leak pass is involved, for which when a leak current increases, a current runs at the off time. To cope with this, Japanese Laid-Open No. 2002-57155 proposes a method of improving a leak current, in which after formation of a film made of tantalum pentaoxide or titanium oxide, the film is crystallized by thermal treatment in an non-oxidative gas at a temperature of not lower than 700° C.